In the past, there has been extensive activity in the field of papermaking to discover ways of imparting softness and absorbency to sanitary paper without excessively degrading its strength. Typically, such properties are imparted by creping the webs from a dryer surface, such as a Yankee dryer, with a creping blade. Such a process disrupts and breaks many of the interfiber bonds in the paper web which are formed during the drying thereof by the hydrate bonding process associated with papermaking. However, these interfiber bonds are the principal source of strength in conventional paper webs, so an increase in absorbency and softness is accompanied by a loss of strength.
The effectiveness of creping can generally be improved by increasing the adhesion of the web to the dryer surface. Such increased adhesion is conventionally provided by greater drying of the web before removing it with the creping blade. However, the drier the web, the greater will be the cohesive forces between the fibers and the more resistance the web will have to the creping action. Thus, in conventional creping, there is a balance needed between the interfiber cohesive forces and the adhesive forces between the web and the dryer surface to produce the most desirable results.
One form of conventional creping process is known as wet creping, in which the web is creped at a dryness of from about 45% to about 60%. In this dryness range, the web is sufficiently wet to have only a small portion of its final interfiber bonding in effect, which provides an opportunity to substantially increase or decrease the interfiber cohesive forces by varying the dryness at the creping blade. However, reduced interfiber bonding due to higher moisture is accompanied by reduced adhesion forces to the creping dryer. Thus, a desirable decrease in one force results in an undesirable decrease in the other force.
The creping effect from conventional wet creping has been studied extensively and has been found to be quite predictable. By varying the dryness level of the web at creping, different absorbency and strength results can be obtained over a wide range. The relationship between absorbency and tensile strength of the creped web has been found to be very consistent, and when the two properties are plotted against each other on a graph for varied examples, a line can be drawn which generally joins the values all together. This line shall be identified herein as the "Conventional Absorbency/Strength Curve."
It would be very desirable to modify the results of wet creping to provide an absorbency/strength relationship which is above (more absorptive) the Conventional Absorbency/Strength Curve. The desired modification could be accomplished by reducing the interfiber cohesive forces in the web more than the adhesive forces between the web and the creping dryer surface are reduced. The preferred method of the invention accomplishes this by spraying water onto the backside of the web just prior to creping and before the moisture has time to penetrate sufficiently into the thickness of the web to significantly reduce the adhesive forces. Prior art techniques for applying water to a paper web at the creping dryer were done either at the point where the web is pressed to the dryer or just before. In either case, it was applied sufficiently far in advance of creping to allow the water to spread uniformly through the thickness of the web, providing the equivalent of conventional wet creping at a higher moisture content due to the added water. In many of such applications, the water was used as a carrier for applying a chemical additive to the web, such as a flame retardant.